Environmental interests and an increasing demand for diesel fuel, for example, in Europe, can encourage fuel producers to employ more intensively available renewable sources. In the manufacture of diesel fuels based on biological raw materials, an interest has concentrated on vegetable oils and animal fats comprising triglycerides of fatty acids. Long, straight and mostly saturated hydrocarbon chains of fatty acids can correspond chemically to the hydrocarbons present in diesel fuels. However, neat vegetable oils can display inferior properties, for example, extremely high viscosity and poor stability, and therefore their use as such in transportation fuels as components can be limited.
Approaches for converting vegetable oils or other fatty acid derivatives into liquid fuels comprise, for example, transesterification into fatty acid methyl ester (FAME). Unfortunately, the obtained product can offer poor low-temperature properties and can result in undesirably high emissions of NOx in comparison to diesel fuels.
One approach is to convert the fatty acids or their esters to hydrocarbons by deoxygenation reactions. The deoxygenation of oils and fats based on biological material may be carried out by catalytic hydroprocessing, such as hydrocracking, or in a more controlled manner using hydrotreating conditions. During hydrotreating, for example, hydrodeoxygenation, oxygen containing groups are reacted with hydrogen in the presence of a catalyst, and oxygen is removed through formation of water.
However, due to, for example, a high amount of phosphorus and metal impurities in vegetable or animal oil or fat ash containing material will be formed, the probability of side reactions may be increased and deactivation of the catalyst may be likely. Metals in biological oil may additionally form metal soaps which promote plugging of preheating section and decrease catalyst activity and operating life. Therefore, it can be desirable to decrease the content of impurities in the crude oil to be treated prior to further processing.
The impurity quality and quantity of biological oils and/or fats can vary considerably in feedstock from varying origin. Different impurities can be removed by different purification procedures. For example, animal fats can provide the most challenging source material which can be very difficult to purify. On the other hand, animal fat can be a highly desirable feedstock for fuel production, since animal fat is considered a waste stream which, for example, can require or benefit from proper treatment to be discharged. Animal fat has shorter fatty acids chain lengths compared to many vegetable oils which can result in excellent properties in renewable fuels produced by hydrogenation thereof.
Biological raw material can contain, in addition to metals and phosphorus, metal compounds, organic nitrogen and sulfur which can act as catalyst inhibitors and poisons, for example, reducing the catalyst service life and, for example, making more frequent catalyst regeneration or replacement desirable. Metals in biological oils and/or fats can build up on catalyst surface and change activity and selectivity of the catalyst. Moreover, metals can promote side reactions such as ash forming and blocking of catalyst. These kinds of phenomena can increase the pressure drop over catalyst beds and further decrease the activity of the catalysts. For example, metals such as Na, Ca, Mg and Fe can be detrimental and it can be desirable to remove same as efficiently as possible.
A desire for purification of biological oils and fats before subjecting them to hydrotreatment and formation of renewable fuel components is recognized in several publications. US 2006/0264684 describes several pre-treatment methods for purification of biological oils. US 2007/0010682 suggests degumming and/or bleaching of biological oils in order to reduce the phosphorous and total metal content of the biological oils prior to the hydrotreatment process.